1. Field of the Invention
The present invention is directed, generally, to a method of forming poly-(3-substituted) thiophenes and, more particularly, to a method of forming head-to-tail coupled regioregular poly-(3-substituted) thiophenes.
2. Descrition of the Invention Background
Poly-(3-substituted) thiophenes (PTs) represent a class of polymer that are highly processable and exhibit relatively high environmental stability, thermal stability, and electrical conductivity. As a result, these materials have been found to be promising candidates for numerous applications, ranging from electronic and optical devices, such as field-effect transistors, sensors, light-emitting diodes (LEDs), rechargeable batteries, smart cards, and non-linear optical materials, to medical applications, such as artificial muscles. The discovery of additional applications and new technologies is subject, in large part, to molecular designers' ability to control the structure, properties, and function of PTs chemical synthesis. Those in the art have come to recognize that structure plays an important, if not critical role, in determining the physical properties of conducting polymers.
Because of its asymmetrical structure, the polymerization of 3-substituted thiophene produces a mixture of PTs structures containing three possible regiochemical linkages between repeat units. The three orientations available when two thiophene rings are joined are the 2,2', 2,5', and 5,5' couplings. When application as a conducting polymer is desired, the 2,2' (or head-to-head) coupling and the 5,5' (or tail-to-tail) coupling, referred to as regiorandom couplings, are considered to be defects in the polymer structure because they cause a sterically driven twist of thiophene rings that disrupt conjugation, produce an amorphous structure, and prevent ideal solid state packing, thus diminishing electronic and photonic properties. A representation of regiorandom couplings is shown in FIG. 2. The steric crowding of the solubilizing groups in the 3 position leads to loss of planarity and less .pi. overlap. In contrast, the 2,5' (or head-to-tail (HT) coupled) regioregular PTs can access a low energy planar conformation, leading to highly conjugated polymers that provide flat, stacking macromolecular structures that can self-assemble, providing efficient interchain and intrachain conductivity pathways. The electronic and photonic properties of the regioregular materials are maximized. A representation of regioregular coupling is shown in FIG. 1.
Various methods have been employed to synthesize 2,5' regioregular PTs, some of which are described by R. D. McCullough, "The Chemistry of Conducting Polythiophenes", Advanced Materials, Vol. 10, No. 2, pages 93-116 (1998), which is incorporated herein by reference in its entirety. The Advanced Materials article describes early and well known methods previously published by R. D. McCullough and R. S. Loewe; (the "McCullough method") and T. A. Chen and R. D. Rieke (the "Rieke method"). More recent approaches to regioregular synthesis are described using chemistry developed by Stille, A. Iraqi and G. W. Barker, J. Mater. Chem., Vol. 8, pages 25-29 (1998), and Suzuki, S. Guillerez and G. Bidan, G. Synth. Met., Vol. 93, pages 123-126, which are incorporated herein by reference. All four methods produce polythiophenes with a high percentage of HT couplings, in the range of 95% or higher.
The McCullough method, developed by one of the applicants of the present invention, synthesizes HT-poly(3-alkylthiophenes) (PATs) at or near about 100% couplings. As illustrated below, this method regiospecifically generates 2-bromo-5-(bromomagnesio)-3-alkylthiophene from a monomer, which is polymerized with catalytic amounts of Ni(dppp)Cl.sub.2 (1,3-diphenylphosphinopropane nickel(II) chloride) using Kumada cross-coupling methods. The McCullough method can be illustrated as follows: ##STR1##
The Rieke method differs from the McCullough method primarily in the synthesis of the asymmetric organometallic intermediate. As illustrated below, a 2,5-dibromo-3-alkylthiophene is added to a solution of highly reactive "Rieke zinc" (Zn*) to form a mixture of the isomers, 2-bromo-3-alkyl-5-(bromozincio) thiophene and 2-(bromozincio)-3-alkyl-5-bromothiophene. The addition of Ni(dppe)Cl.sub.2 (1,2-bis(diphenylphosphino)ethane nickel(II) chloride), a nickel cross-coupling catalyst, leads to the formation of a regioregular HT-PATs. The Rieke method can be illustrated as follows: ##STR2##
Other methods, such as the Stille and Suzuki methods, use a Pd catalyst rather than a Ni catalyst. The Stille method can be illustrated as follows: ##STR3##
The Suzuki method can be illustrated as follows: ##STR4##
Despite the efforts by those skilled in the art to improve HT coupling techniques, the synthetic procedures heretofore described have significant drawbacks. For example, the McCullough method requires highly purified starting materials, the most important of which is the monomer, 2-bromo-3-alkylthiophene. The need for purity adds to the cost of the synthesis. The Rieke method includes the easy to purify 2,5-dibromo-3-alkylthiophene as the starting material (because the compound is the highest boiling fraction in the crude mixture in its preparation), but requires the non-trivial preparation of Rieke zinc via alkali metal reduction of zinc halides in an inert environment. The Rieke zinc is very difficult to produce and, therefore, very costly. Both the Stille and Suzuki methods require an extra processing step in their synthesis, thereby decreasing their manufacturing efficiency and flexibility. All of the above illustrated synthesis reactions require cryogenic temperatures at some point during the synthesis, and long polymerization times of 12 to 24 hours or longer. In addition, there have been no reports that any of these known methods have been used for the large scale synthesis of HT-PTs.
Accordingly, a new method for the preparation of HT-poly-(3-alkylthiophenes) is needed that is efficient, and economical, provides greater manufacturing flexibility, and is suited for use in large scale industrial processes.